A dynamic speaker unit is known as a speaker that converts an electrical signal into sound. The dynamic speaker unit is configured with a permanent magnet, a voice coil, a diaphragm, and the like. In the dynamic speaker unit, the phases of sound are opposite at the front and the back of the diaphragm. Therefore, when sound from the back comes to the front, the sound from the front and the sound from the back cancel with each other, resulting in a weak sound. In order to block sound coming from the back to the front, the dynamic speaker unit is built into an enclosure to be used. In this case, it is necessary that the air in the enclosure should not interfere with motion of the diaphragm. The enclosure therefore tends to increase in size.
Piezoelectric speakers using piezoelectric materials such as lead zirconate titanate and polyvinylidene fluoride resin have been developed as thin speakers (see, for example, Patent Documents 2 and 3). However, sound in a low frequency region is difficult to be produced because of high rigidity of the piezoelectric materials. In this respect, Patent Documents 4 and 5 disclose a transducer in which a pair of electrodes are arranged on the front and back surfaces of an elastomer dielectric layer. In transducers of this type, when voltage applied between the electrodes is increased, an electrostatic attraction between the electrodes increases, and the dielectric layer interposed between the electrodes is compressed in the thickness direction. The thickness of the dielectric layer is thus reduced. When the thickness of the dielectric layer is reduced, the dielectric layer accordingly expands in the surface direction. On the other hand, when voltage applied between the electrodes is reduced, an electrostatic attraction between the electrodes decreases, and thus, the compression force on the dielectric layer decreases. This increases the thickness of the dielectric layer. When the thickness of the dielectric layer is increased, the dielectric layer accordingly contracts in the surface direction. The transducer can be used as a speaker by utilizing the change in thickness of the dielectric layer with voltage.